In long distance fiber optic communication systems it is important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems with the system.
Prior art monitoring techniques include the use of a testing system which generates a test signal and modulates the test signal onto a single channel (or wavelength) with the transmitted data signal. For example, the data signal may be amplitude modulated by the test signal. A loop-back coupler within an optical amplifier or repeater located downstream is used to return a portion of the transmitted signal (data signal plus test signal modulation) to the testing system. The testing system then separates the test signal from the data signal and processes the test signal to examine the health of the transmission system. U.S. Pat. Nos. 4,586,186 and 4,633,464 to C. Anderson et al. discloses a similar technique to modulate test response information from a repeater onto the main data signal to monitor the health of the system.
One problem that arises from the use of an amplitude modulated test signal is that crosstalk is generated between the test signal and data channels. It has been determined that the crosstalk increases as the frequency separation between the test signal and the data channels increase. That is, data channels nearest the test signal exhibit the least crosstalk while data channels farthest from the test signal exhibit the greatest crosstalk. Moreover, crosstalk occurs among data channels that are both greater and less than the frequency of the test signal. Based on these observations, the source of the crosstalk is Raman gain that results in a transfer of power between the test signal and the data channels.
Accordingly, it would be desirable to reduce the cross talk that arises between the test signal and the data channels in a wavelength division multiplexed optical communication system.